Bromonapthalenes have no known industrial use or application, but have been identified as contaminants of Firemaster BP-6, the toxic mixture of polybrominated biphenyls used as a fire retardant and involved in a major episode of environmental poisoning in Michigan. Structurally related to other halogenated aromatic xenobiotics, their toxicity and disposition seem to vary with the position of bromination. This work has studied the chemical disposition of a mixture of 2 hexabromonapthalenes (HBNs), previously identified as a single isomer, 1,2,3,4,6,7-HBN. The compound is incompletely absorbed after an oral dose. After iv treatment over 50% of the dose is excreted as metabolites within 3 days. However, the remainder of the dose seems to be extremely persistent, over 25% remaining in the liver after 35 days. These disposition results led to proof of the presence of two isomers by high resolution NMR, present in a ratio of 65:35 which have been identified as 1,2,3,4,6,7- and 2,3,4,5,6,7 -HBN. The difference in the fate of the two isomers has been proven by isolation and characterization by high resolution NMR of the HBN remaining in the liver 10 days after treatment. While the HBN dosed was in an isomeric ratio of 65:35 (1,2,3,4,6,7-:2,3,4,5,6,7-), the HBN in the liver 10 days after oral treatment was in the ratio of 20:80. The toxicity of this HBN mixture was examined in mice. A single oral dose as high as 1000 mg/kg had no toxic effects. However, repeat dose toxicity was detected at doses as low as 5 mg/kg for 7 days. The toxic response was toxicity similar to that seen for TCDD and related compounds. A complete teratology study was carried out and the teratogenic response was identical to that observed with TCDD, with the main endpoints being kidney anomalies and cleft palate at doses as low as 1 mg/kg.